The Toronto Stock Exchange's S&P/TSX fell 39.78 points, or 0.24 percent, to 16,488.12.
Leading the index were Nexgen Energy Ltd , up 10.5 percent, Aritzia Inc , up 7.1 percent, and Martinrea International Inc , higher by 4.9 percent.
Lagging shares were CannTrust Holdings Inc , down 17.3 percent, Canopy Growth Corp, down 8.0 percent, and Cronos Group Inc, lower by 6.2 percent.
On the TSX 117 issues rose and 114 fell as a 1-to-1 ratio favored advancers. There were 9 new highs and 3 new lows, with total volume of 160.3 million shares.
The most heavily traded shares by volume were Aurora Cannabis Inc , Canntrust Holdings Inc and Toronto-dominion Bank.
The TSX's energy group fell 0.17 points, or 0.1 percent, while the financials sector slipped 0.84 points, or 0.3 percent.
West Texas Intermediate crude futures rose 0.07 percent, or $0.04, to $60.24 a barrel. Brent crude rose 0.42 percent, or $0.28, to $66.8.
The TSX is up 15.1 percent for the year.